An animal house ventilation system 1 comprises a roof 2 supporting exhaust chimneys 4 and vents 3 in walls of the house. Some chimneys 4 have both baffles 6 and exhaust fans 7, and other chimneys 5 have only baffles 8 without fans. A controller 10 is linked with sensors including temperature, pressure, ammonia and carbon dioxide and controls the vents, the exhaust fans, and the exhaust baffles. The controller causes the system to operate as a natural ventilation system utilizing the vents and all of the chimneys in the roof as openings for ventilation until a point is reached where there is not enough air movement through the house under natural ventilation conditions. At this stage the fans are started at a low speed and the baffles in the chimneys without fans are closed to ensure that there is no downdraught. The inlet vents continue to open at the same time the fans speed are increased, and continue to increase in speed after the inlet vents have fully opened to keep the temperature at a required level.
